Laser ablation for the environmentally beneficial removal of surface coatings

ABSTRACT

A laser-based method of removing a coating from a surface comprises directing a laser pulse to a first position on the surface, removing the coating from the first position by rapidly elevating a surface temperature of the first position using the laser pulse and thereby disassociating the coating from the surface and collecting the disassociated coating. In some embodiments, the coating comprises an environmentally harmful substance such as Hexavalent Chromium. In some embodiments, the coating comprises Diamond-Like Carbon (DLC), Vitrified Contaminant Material (CMAS). The disassociated coating is collected by a waste collector.

RELATED APPLICATIONS

This application claims priority of U.S. provisional application, Ser.No. 61/604,368, filed Feb. 28, 2012, and entitled “Laser Ablation forthe Environmentally Beneficial Removal of Three Substances;Diamond-Like-Carbon (DLC), Vitrified Contaminant Materials (CMAS), andPaints/Primers containing Hexavalent Chromium,” which is incorporatedherein by reference.

FIELD OF THE INVENTION

The invention relates to ablating a coating using a laser. Inparticular, the invention relates to a laser apparatus that operates atconditions for removal of coatings such that the process isenvironmentally beneficial in comparison to other removal mechanisms.

BACKGROUND OF THE INVENTION

Laser-based coating removal systems use pulses of light from high powerlasers to ablate or vaporize the paint or other coating from a surface.Ablation is the thermochemical disassociation, vaporization, and removalof material because of the incident convective and radiant energy. FIG.1 illustrates the surface action phenomena during a typical ablationprocess.

Commercially-available lasers have the capability to generate pulses ofradiant energy for suitable duration to achieve ablation conditionswithout significant convective heat transfer. Specifically,laser-induced ablation can be limited to the material surface throughthe appropriate selection of laser wavelength and active control of theradiative heat transfer rate and duration. The important application isthe removal of the coatings and finishes from substrate structures thatmust be protected and reused. The radiative energy input must besignificant, but the heat conduction into the substrate must beminimized. The amount of heat conduction into the substrate is affectedby the radiant energy and the material surface of the substrate. Thewavelength, irradiance, and pulse width of the radiant energy impact theamount of heat conduction into the substrate. The substrate properties,including transmissivity, absorptivity, emissivity, density, specificheat, thermal conductivity, and ablation onset thermochemistry (i.e.melt, vaporization, and surface chemical reaction temperatures), alsoaffect the heat conduction into the substrate. m

Wavelength is an important parameter because it must be selected tominimize the transmissivity of the radiant energy into the substrate.Thus, the radiant energy is absorbed at the surface. A high absorptivityof the material surface is also valuable, as this reduces the amount oflaser energy needed to achieve a desired ablation rate. In manyapplications, such as metal substrates below a surface coating, lowsubstrate absorptivity is also advantageous, because this limits thepotential for ablation after the surface material is removed.Alternatively, substrate ablation for highly absorbing substrates mayneed to be limited by the use of active surface recognition prior to theinitiation of the laser radiant energy pulse.

SUMMARY OF THE INVENTION

Embodiments of the present invention are directed to a laser-basedcoating removal method and system to remove an identified coating in aprocess that is environmentally beneficial in comparison to otherremoval mechanisms. The method comprises directing a laser pulse to afirst position on a surface having an identified coating. The laserpulse rapidly elevates the surface temperature at the first position andcauses the coating to disassociate from the surface. After the coatinghas disassociated from the surface it is able to be collected and safelydisposed.

In one aspect a laser-based method of removing a coating from a surfacecomprises directing a laser pulse to a first position on the surface,removing the coating from the first position by rapidly elevating asurface temperature of the first position using the laser pulse andthereby disassociating the coating from the surface and collecting thedisassociated coating. In some embodiments the coating comprises anenvironmentally harmful substance such as Hexavalent Chromium. In someembodiments the coating comprises Diamond-Like Carbon (DLC) or Vitrified

Contaminant Material (CMAS). In some embodiments, the surfacetemperature of the first position is elevated to the oxidationtemperature of the DLC and it is converted to vapor-phase carbon dioxidebefore it is collected. In further embodiments, the surface temperatureof the first position is elevated so that the components of the CMAS aredisassociated from the surface by disrupting the chemical bonds and theadmixture is disaggregated. In still further embodiments, the surfacetemperature of the first position is elevated to a temperature at whichthe hexavalent chromium is disassociated from the surface and reduced toa particulate form of trivalent chromium. In some embodiments, thedisassociated coating is collected by a waste collector. In furtherembodiments, the heat conduction into the surface from the laser pulseis limited. In some embodiments, the disassociation occurs essentiallyindependent of the laser wavelength. In some embodiments, the laserpulse is moved to a second position in order to remove a coating fromthe second position.

In another aspect, a laser-based coating removal system comprises asurface comprising a coating and a laser based removal apparatuscomprising a laser source to provide a laser pulse to a first positionon the surface, wherein the laser source removes the coating from thesurface by rapidly elevating a surface temperature of the first positionusing the laser pulse and thereby disassociating the coating from thesurface. In some embodiments, the coating comprises an environmentallyharmful substance such as Hexavalent Chromium. In some embodiments, thecoating comprises Diamond-Like Carbon (DLC) or Vitrified ContaminantMaterial (CMAS). In some embodiments, the surface temperature of thefirst position is elevated to the oxidation temperature of the DLC andit is converted to vapor-phase carbon dioxide before it is collected. Infurther embodiments, the surface temperature of the first position iselevated so that the components of the CMAS are disassociated from thesurface by disrupting the chemical bonds and the admixture isdisaggregated. In still further embodiments, the surface temperature ofthe first position is elevated to a temperature at which the hexavalentchromium is disassociated from the surface and reduced to a particulateform of trivalent chromium. In some embodiments, the disassociatedcoating is collected by a waste collector. In some embodiments, the heatconduction into the surface from the laser pulse is limited. In furtherembodiments, the disassociation occurs essentially independent of thelaser wavelength.

In a further aspect, a method for removing Diamond-Like Carbon (DLC)from a surface comprises directing a laser pulse to a first position onthe surface, elevating a surface temperature of the first position usingthe laser pulse to an oxidation temperature of the DLC therebyconverting it to vapor-phase carbon dioxide and collecting thedisassociated DLC from the surface.

In still a further aspect, a method of removing Vitrified ContaminantMaterial (CMAS) from a surface comprises directing a laser pulse to afirst position on the surface, elevating a surface temperature of thefirst position using the laser pulse to disassociate the components ofthe CMAS from the surface and disaggregating the admixture andcollecting the disassociated CMAS from the surface.

In another aspect, a method for removing Hexavalent Chromium from asurface comprises directing a laser pulse to a first position on thesurface, elevating a surface temperature of the first position using thelaser pulse to a temperature at which the hexavalent chromium isdisassociated from the surface and reduced to a particulate form oftrivalent chromium and particulate effluent, and collecting thetrivalent chromium and particulate effluent from the surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the surface interaction phenomena during an ablationprocess.

FIG. 2 illustrates a laser-based method for removing an identifiedcoating from a surface according to some embodiments.

FIG. 3 illustrates a laser-based method for removing an identifiedcoating from a surface according to some embodiments.

FIG. 4 illustrates a laser-based method for removing an identifiedcoating from a surface according to some embodiments.

FIG. 5 illustrates a laser-based system for removing an identifiedcoating from a surface according to some embodiments.

Embodiments of the invention are described relative to the several viewsof the drawings. Where appropriate and only where identical elements aredisclosed and shown in more than one drawing, the same reference numeralwill be used to represent such identical elements.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Reference will now be made in detail to implementations of a laser-basedcoating removal system and method as illustrated in the accompanyingdrawings. The same reference indicators will be used through thedrawings and the following detailed description to refer to the same orlike parts. In the interest of clarity, not all of the routine featuresof the implementations described herein are shown and described. It willalso be appreciated that in the development of any such actualimplementation, numerous implementation-specific decisions be made toachieve the developer's specific goals, such as compliance withapplication and business related constraints, and that these specificgoals will vary from one implementation to another and from onedeveloper to another. Moreover, it will be appreciated that such adevelopment effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking of engineering for those ofordinary skill in the art having the benefit of this disclosure.

Embodiments of the present invention are directed to a laser-basedcoating removal method and system to remove an identified coating from asurface in a process that is environmentally beneficial in comparison toother removal mechanisms. The system comprises a laser source to providea laser light, a routing element coupled to the laser source andconfigured to direct the laser light onto a target region of the surfacethereby removing the coating from the target region. In someembodiments, the system comprises a detection means to detect thecoating as the coating disassociates from the target region of thesurface. In some embodiments, the system comprises a waste collector forcollecting the coating that has been removed.

A laser-based method for removing an identified coating from a surfaceis able to remove the identified coating in an environmentallybeneficial manner. A laser pulse is directed to the surface in order toraise the temperature on the surface of the coating. When thetemperature of the surface reaches a critical point, the coatingdisassociates from the surface and is able to be collected and safelydisposed. Using a laser-based method avoids media-blast or chemicallybased processes that are time consuming and generate significant toxicwaste. The laser-based method rapidly raises the temperature of thesurface to its critical point under strictly controlled operatingparameters in order to induce the desired effects.

In some embodiments, the system and method is used to selectively removethe identified coatings Diamond-Like Carbon (DLC), VitrifiedContaminants (CMAS), and primers and paints containing hexavalentchromium in an environmentally beneficial manner. However, the system isable to selectively remove other environmentally harmful chemicals asdesired.

Diamond-Like Carbon (DLC)

DLC exists in different forms of carbon atoms that display some of thetypical properties of diamond. DLC is applied as coatings to othermaterials in order to benefit from the diamond-like properties.Particularly, the value of a DLC coating accrues from its ability toprovide properties of diamond to the surface of almost any material.These properties include hardness, wear, resistance, x-ray transparency,high thermal conductivity, and low friction. For example, a DLC filmfriction coefficient against polished steel ranges from 0.05-0.20. DLCis often applied in order to reduce the abrasive wear in high-loadbearing surfaces, cutting tools, and press molds.

Removal of the DLC coating for substrate inspection and repair poses asignificant challenge because of the coating's hardness and chemicalresistance. Consequently, DLC film removal is sometimes attempted usingan oxygen-dominated plasma beam to generate an appropriate ionizedoxygen gas flow rate that etches the DLC film in a manner proportionalto the treatment time. Alternatively, DLC may be removed byelectrolyzing the film in an acid solution. However, these methods arelimited by the processing rate, incomplete removal of the DLC film,access to confined spaces, field operability and/or waste chemicalgeneration.

A laser-based method for DLC removal is able to remove DLC by impinginghigh irradiance laser pulses on a target DLC surface in an oxygen-richenvironment. The laser pulse contacts the surface and immediatelyelevates the DLC to its carbon dioxide temperature, thereby rapidly andcleanly converting the DLC to vapor phase carbon dioxide. When the DLChas been converted to the vapor phase carbon dioxide it is able to becollected and safely disposed. Additionally, the limited heat conductioninto the substrate provided by the laser process protects the coatedsurface from damage. In some embodiments, closed-loop feedback controlimmediately suppresses the beam when the DLC coating is completelyremoved.

FIG. 2 illustrates a laser-based method of removing DLC from a surfacein accordance with some embodiments. The method begins in the step 210.In the step 220, a laser pulse is directed to a position of the DLC on asurface of the substrate. In the step 230, a surface temperature of theposition is elevated using the laser pulse to an oxidation temperatureof the DLC. When the DLC reaches the oxidation temperature it isconverted to vapor phase carbon dioxide and disassociates from thesurface of the substrate. After the DLC disassociates from the surfaceit is able to be collected in the step 240 and disposed. As describedabove, the laser pulse contacts the surface and immediately elevates theDLC to vapor phase carbon dioxide, disassociating it from the surface ofthe substrate. The limited heat conduction into the substrate protectsthe coated surface from damage.

Vitrified Contaminant Material (CMAS)

Operation of gas turbine engines in typical environments leads toingestion of significant mounts of sand, dust, volcanic ash, and otheratmospheric contaminants and containing oxides of Calcium, Magnesium,Aluminum, and Silicon (CMAS). In the high temperature, high pressuresections of these engines, the contaminants vitrify and accumulate onthe turbine flow surfaces. The accumulations cannot be readily removedwith conventional media blast or chemical exposure methods.Consequently, the conventional cleaning methods entail dismantling theengine, removing the contaminated component and then exposing thesurfaces to a highly-concentrated acid bath for a protracted period. Theacid baths are a toxic exposure hazard to personnel and the environmentwhile only partially removing the contaminants.

A laser-based method is able to remove the CMAS quickly and completelyfrom exposed surfaces and small openings such as bleed air holes, pores,cracks, and crevices. In some embodiments, laser-based cleaning may beaccomplished in situ or on subassemblies removed from the engine. A highirradiance laser pulse produces a rapid temperature rise of the surfaceof the CMAS and simultaneously disassociates the CMAS components,disrupts the amorphous chemical bonds and disaggregates the admixtures.Once the CMAS is disassociated from the surface, purge air is able to bedirected towards the point of ablation and adjacent vacuum collectorsresult in a rapid and thorough cleaning of the surfaces within the focalrange of the laser delivery device.

FIG. 3 illustrates a method of removing vitrified contaminant material(CMAS) from a surface in accordance with some embodiments. The methodbegins in the step 310. In the step 320, a laser pulse is directed to aposition of the CMAS on the surface of the substrate. Then, in the step330, a surface temperature of the position is elevated using the laserpulse to a temperature at which the CMAS disassociates from the surfaceand disaggregates. After the CMAS disassociates from the surface it isable to be collected in the step 340 and disposed. In some embodiments,the CMAS is collected by utilizing purge air and vacuum collectors todirect the CMAS to a waste collector. As described above, the laserpulse produces a rapid temperature rise of the surface of the CMAS andsimultaneously disassociates the CMAS components, disrupts the amorphouschemical bonds and disaggregates the admixtures.

Hexavalent Chromium Removal and Conversion to Trivalent

Paints and primers such as used in numerous applications includecompounds containing hexavalent chromium, often in the form of zincchromate (ZnCr0₄). ZnCr0₄ is a chemical compound containing the chromateanion, appearing as an odorless yellow solid powder and is usedindustrially in chromate conversion coatings. Its use as a corrosionresistant agent was applied to aluminum alloy parts first in commercialaircraft, then in military aircraft. During the 1940's and 1950's it wasused as paint in the wheel wells of retractable landing gear of militaryaircraft to protect the aluminum from corrosion. It is also used on manysystems as a primer.

Chromates such as ZnCr0₄ have been linked to occupational diseases,specifically dermatitis, nasal irritation, and lung cancer. As a result,numerous organizations have directed abatement programs and with varyingdegrees of success. Removal of ZnCr0₄ coatings using standard abrasivemethods such as media blast or sanding, or the use of solvents resultsin an unacceptable level of operator exposure and generates large toxicwaste streams.

A laser-based ablation method for zinc chromate rapidly removes the zincchromate coat without damaging the substrate material whilesimultaneously converting the hexavalent chromium to less toxicparticulate forms of trivalent chromium (Cr₂O₃). The less-toxicparticulate effluent is then able to be collected by conventionalfiltration techniques. Impingement of a high irradiance laser pulse onthe surface of a coating with ZnCr0₄ rapidly raises the surfacetemperature of the coating to a value at which the hexavalent chromiumis reduced to its triavalent form. For example, in some embodiments, theelevated temperature created by the laser pulse drives the followingequilibrium chemical reaction completely to the right.

4ZnCr0₄+ . . . →4ZnO (solid)+2Cr₂O₃ (solid)+3O₂+ . . .

FIG. 4 illustrates a method of removing hexavalent chromium from asurface of a substrate in accordance with some embodiments. The methodbegins in the step 410. In the step 420, a laser pulse is directed to aposition of the hexavalent chromium on a surface of a substrate. In thestep 430, the temperature at the position is elevated using the laserpulse to a temperature at which the hexavalent chromium is disassociatedfrom the surface and reduced to trivalent chromium and byproducts. Afterthe hexavalent chromium is disassociated and reduced it is able to becollected in the step 440 and safely disposed. In some embodiments, theless-toxic trivalent chromium and byproducts is collected byconventional filtration techniques. As described above, the laser-basedablation method for zinc chromate rapidly removes the zinc chromate coatwithout damaging the substrate material while simultaneously convertingthe hexavalent chromium to less toxic particulate forms of trivalentchromium (Cr₂O₃).

FIG. 5 illustrates a laser-based coating removal system for theenvironmentally beneficial removal of surface coatings in accordancewith some embodiments. The system 500 comprises a laser source 502, arouting element 504, and a controller 506 for removing a coating 98 froma surface 99. The laser source 502 is directed to the coating 98 on thesurface 99 by the routing element 504. Specifically, the routing element504 directs the laser pulse 97 to the surface 99 in order to remove thecoating 98 from the surface 99, as described above. When the laser pulse97 contacts the surface 99, the temperature of the surface isimmediately elevated to a critical temperature at which the coating 98is disassociated from the surface 99. In some embodiments, the routingelement 504 comprises one or more beam splitters and/or scanning opticswhich direct the laser pulse 97 to the surface.

As shown in FIG. 5, in some embodiments, the system 500 comprises adetection means 508 for detecting the removal of the coating 98 from thesurface 99 and a waste collector 510 for collecting the waste byproductsresulting from the laser pulse 97 contacting the surface 99. In someembodiments, the waste collector 510 includes a local storage forstoring the collected ablated waste byproduct. Alternatively, the wastecollector 510 acts as a waste removal apparatus and is coupled to anexternal waste receptacle.

In some embodiments, the routing element 504 and the detection means 508are of the type as described in the co-owned U.S. Pat. No. 7,633,033 andentitled “Color Sensing for Laser Decoating”, which is herebyincorporated by reference. In some embodiments, the waste collector 510of the type as described in the co-owned U.S. Pat. No. 7,009,141 andentitled “Laser Scanning Head with Rotary Scanning Coaxial RefractiveOptics”, which is also hereby incorporated by reference.

The laser pulse is able to comprise a fluence in the range of 1-10Joules per square centimeter (J/cm²) depending on the coating and thesubstrate. In some embodiments, the irradiance of the laser pulse is inthe range of 6-60 MegaWatts per square cm (MW/cm²). Particularly, thelaser pulse power is sufficient in combination with the laser optics andbeam quality to achieve the desired irradiance. Additionally, the laserablation is achieved essentially independent of the laser wavelengthbecause Diamond-Like Carbon (DLC), Vitrified Contaminants (CMAS), andchromate-containing coatings have low transmissivity for the typicalindustrial laser wavelengths in the range of 0.3 μm to 10.6 μm. Further,the method is minimally affected by the laser pulse duration.Specifically, in some embodiments, the method uses a laser with adelivered average power of 350 W providing a fluence of 5 J/cm² andirradiance of 35 MW/cm².

In its application, a laser-based method of removing a coating from asurface is able to remove an identified coating from the surface and inan environmentally beneficial manner. Particularly, a laser pulse isable to be directed to a position on a surface in order to elevate thesurface temperature to a critical temperature at which a coating isdisassociated from the surface. Once the coating disassociates from thesurface it is able to be collected and safely disposed. Thus, it isclear that the coating removal system described herein has numerousadvantages. Specifically, the coating removal system is able toefficiently remove environmentally harmful coatings on a surface in aprocess that is environmentally beneficial in comparison to otherremoval mechanisms Particularly, the laser-based method rapidly raisesthe temperature of the surface to its critical point under strictlycontrolled operating parameters in order to induce the desired effects.As a result, the laser-based method avoids media-blast or chemicallybased processes that are time consuming and generate significant toxicwaste.

The present invention has been described in terms of specificembodiments incorporating details to facilitate the understanding of theprinciples of construction and operation of the invention. Suchreference herein to specific embodiments and details thereof is notintended to limit the scope of the claims appended hereto. It will beapparent to those skilled in the art that modifications may be made inthe embodiment chosen for illustration without departing from the spiritand scope of the invention.

What is claimed is:
 1. A laser-based method of removing a coating from asurface comprising: a. directing a laser pulse to a first position onthe surface; b. removing the coating from the first position by rapidlyelevating a surface temperature of the first position using the laserpulse and thereby disassociating the coating from the surface; and c.collecting the disassociated coating.
 2. The method of claim 1 whereinthe coating comprises an environmentally harmful substance such asHexavalent Chromium.
 3. The method of claim 1 wherein the coatingcomprises Diamond-Like Carbon (DLC), Vitrified Contaminant Material(CMAS).
 4. The method of claim 3 wherein the surface temperature of thefirst position is elevated to the oxidation temperature of the DLC andit is converted to vapor-phase carbon dioxide before it is collected. 5.The method of claim 3 wherein the surface temperature of the firstposition is elevated so that the components of the CMAS aredisassociated from the surface by disrupting the chemical bonds and theadmixture is disaggregated.
 6. The method of claim 2 wherein the surfacetemperature of the first position is elevated to a temperature at whichthe hexavalent chromium is disassociated from the surface and reduced toa particulate form of trivalent chromium.
 7. The method of claim 1wherein the disassociated coating is collected by a waste collector. 8.The method of claim 1 wherein the heat conduction into the surface fromthe laser pulse is limited.
 9. The method of claim 1 wherein thedisassociation occurs essentially independent of the laser wavelength.10. The method of claim 1 comprising moving the laser pulse to a secondposition in order to remove a coating from the second position.
 11. Alaser-based coating removal system comprising: a. a surface comprising acoating; and b. a laser based removal apparatus comprising i. a lasersource to provide a laser pulse to a first position on the surface,wherein the laser source removes the coating from the surface by rapidlyelevating a surface temperature of the first position using the laserpulse and thereby disassociating the coating from the surface.
 12. Thesystem of claim 11 wherein the coating comprises an environmentallyharmful substance such as Hexavalent Chromium.
 13. The system of claim11 wherein the coating comprises Diamond-Like Carbon (DLC), VitrifiedContaminant Material (CMAS).
 14. The system of claim 13 wherein thesurface temperature of the first position is elevated to the oxidationtemperature of the DLC and it is converted to vapor-phase carbon dioxidebefore it is collected.
 15. The system of claim 13 wherein the surfacetemperature of the first position is elevated so that the components ofthe CMAS are disassociated from the surface by disrupting the chemicalbonds and the admixture is disaggregated.
 16. The system of claim 12wherein the surface temperature of the first position is elevated to atemperature at which the hexavalent chromium is disassociated from thesurface and reduced to a particulate form of trivalent chromium.
 17. Thesystem of claim 11 wherein the disassociated coating is collected by awaste collector.
 18. The system of claim 11 wherein the heat conductioninto the surface from the laser pulse is limited.
 19. The system ofclaim 11 wherein the disassociation occurs essentially independent ofthe laser wavelength.
 20. A method for removing Diamond-Like Carbon(DLC) from a surface comprising: a. directing a laser pulse to a firstposition on the surface; b. elevating a surface temperature of the firstposition using the laser pulse to an oxidation temperature of the DLCthereby converting it to vapor-phase carbon dioxide; and c. collectingthe disassociated DLC from the surface.
 21. A method of removingVitrified Contaminant Material (CMAS) from a surface comprising: a.directing a laser pulse to a first position on the surface; b. elevatinga surface temperature of the first position using the laser pulse todisassociate the components of the CMAS from the surface anddisaggregating the admixture; and c. collecting the disassociated CMASfrom the surface.
 22. A method for removing Hexavalent Chromium from asurface comprising: a. directing a laser pulse to a first position onthe surface; b. elevating a surface temperature of the first positionusing the laser pulse to a temperature at which the hexavalent chromiumis disassociated from the surface and reduced to a particulate form oftrivalent chromium and particulate effluent; and c. collecting thetrivalent chromium and particulate effluent from the surface.